RF-MEMS aims to tune mobile wireless

PORTLAND, Ore. -- As an aspiring fabless semiconductor house, WiSpry Inc. (Irvine, Calif.), recently laid claim to sampling the world's first integrated radio-frequency (RF) micro-electro-mechanical system. The MEMS chip--a digital capacitor array on a CMOS die--can match a cell phone's antenna impedance dynamically, rather than having it set at the factory, saving dropped calls and extending battery life. And that's just the start, says WiSpry, for its forthcoming line of RF-MEMS devices, which the company says eventually will yield the holy grail of RF: software radio--an ultra-wideband communications channel that can be tuned to different bands anywhere in the spectrum.

"In the long run, we want to offer a completely tunable RF-MEMS front end--almost a software radio on-a-chip,"," said co-founder of WiSpry, Jeffrey Hilbert. "But we are beginning at the beginning--that is, with matching antenna impedance with our tunable capacitor arrays."

Keeping the antennas' impedance matched to the RF front-end in a cell phone has become increasingly difficult, as a result of the trend to make antennas internal. Internal antennas tend to couple the RF signal to the user's body, so that just picking up the handset can change the antenna's impedance. Today, electrical engineers design a passive impedance-matching network that can handle the difference between holding the handset and lying it flat on a table, but WiSpry claims its active network can adapt to even unforeseen impedance-matching situations, such operating a handset wearing gloves.

"It sometimes takes a design engineer four weeks to design an impedance-matching network for the antenna on a new cell phone model," said Hilbert. "But with our tunable capacitor array, the impedance matching can be done dynamically, shortening handset design time and improving performance."

WiSpry, a five-year old startup, just decided to start talking about its RF-MEMS development work because it has begun sampling its first chip--an array of 80 digital capacitors on a die, yielding a programmable 10-fold dynamic range in capacitance. The chip won't be ready for customers until next year, but one cell phone handset maker has already signed up for the dynamically matching antenna-impedance application. By adding the tunable digital capacitor array to its cell phone--which can dynamically adjust its impedance in 10 microseconds--a cleaner signal gets through, resulting in fewer dropped calls, extending the range of the mobile wireless service, and lowering the transmission power required, thus extending battery life of the handset.

"It turned out that, 90 percent of the time, the antenna was not perfectly matched to the 50-ohm input impedance of the RF front end," said Hilbert. "But with our tunable capacitor arrays, the impedance can adapt in real-time to always be matched."